1. Field of the Invention
The present invention relates to a catalyst for properly removing nitrogen oxides especially those having a high NO2 content, a catalyst molded product therefor, and an exhaust gas treating method.
Also, the present invention relates to a combined cycle power generation facility having an exhaust gas system provided with a catalyst for properly removing nitrogen oxides especially having a high CO2 content.
Generally, for nitrogen oxides contained in exhaust gas from a boiler etc., NO accounts for 80 to 90% by volume of the nitrogen oxides. As a denitrifying method for such exhaust gas, there is available a method in which a catalyst containing, for example, titanium (Ti), tungsten (W), or vanadium (V) as a main active component is used. In this denitrifying method, exhaust gas is caused to pass through the catalyst, and ammonia is added, by which denitrification is accomplished by the following reaction:4NO+4NH3+O2→4N2+6H2O  (i)
However, of various kinds of exhaust gases exhausted recently, exhaust gases from a gas turbine, diesel engine, and gas engine and further a chemical plant such as a nitrification plant, which have large fluctuations of load, sometimes have a higher NO2 content than the NO content. Under conditions where the NO2 content is high, especially under conditions where the ratio of NO2/NO is 1 or higher, a problem in that the function of the aforementioned catalyst is insufficient arises.
The catalyst for removing nitrogen oxides in accordance with Japanese Patent Provisional Publication No. 1-151940 (No. 151940/1989) filed by the applicant for the present invention has been developed as a catalyst for decomposing NO2. However, this catalyst uses a composite oxide of copper and chromium, so that it is difficult to synthesize the oxide and also it is difficult to demonstrate reproducibility in terms of performance.
From the viewpoint of lowering air pollution, as a method for removing nitrogen oxides generated from a boiler and various combustion furnaces, a denitrifying method by catalytic reduction with ammonia, in which nitrogen oxides are decomposed into nitrogen and water in a contact manner with a catalyst by using ammonia as a reducing agent, has been used industrially as the most economical and efficient method. At present, many plants using this method are being operated.
As a catalyst used in this method, a tungsten oxide, vanadium oxide, molybdenum oxide, iron oxide, etc. are carried on a titanium oxide or an aluminum oxide. In the treatment of exhaust gas containing sulfur oxides, such as heavy oil or coal burning exhaust gas, a catalyst using a titanium oxide as a carrier is superior in terms of resistance to toxicity on sulfur oxides. At present, therefore, a titanium oxide-vanadium oxide-tungsten oxide catalyst is being used as an NOx removing catalyst (hereinafter referred to as denitrification catalyst) in many actual plants because it is excellent in performance and durability.
In a boiler, gas turbine, or gas turbine combined cycle for power generation, a load fluctuating operation is routinely performed according to a change in quantity of generated power. At high load time, there is no hindrance because almost all NOx in exhaust gas exists as NO, but at low load time, almost all NOx in exhaust gas exists as NO2. Therefore, a decrease in NOx removal efficiency of the denitrifier using a denitrification catalyst may frequently pose a problem of smoke color caused by increased concentration of NOx in stack outlet gas.
The inventors of the present invention experimentally analyzed the titanium oxide-vanadium oxide-tungsten oxide catalyst in a reaction between NO and NH3 expressed by the following formula (1), a reaction between NO2/NO mixing gas and NH3 expressed by formula (2), and a reaction between NO2 and NH3 expressed by formula (3).4NO+4NH3+O2→4N2+6H2O  (1)NO+NO2+2NH3→2N2+3H2O  (2)6NO2+8NH3→7N2+12H2O  (3)
As a result, it was revealed that in the case where the mole ratio of NO2/NO is 1 or higher or in the case of only NO2, the NOx removal efficiency of the conventional titanium oxide-vanadium oxide-tungsten oxide catalyst (hereinafter referred to as catalyst A) decreases significantly, and it was found that a titanium oxide-vanadium oxide-tungsten oxide-copper oxide/chromium oxide composite oxide (hereinafter referred to as catalyst B) that takes the place of catalyst A is effective (Japanese Patent Provisional Publication No. 1-151940 (No. 151940/1989)).
In the gas turbine or gas turbine combined cycle, the NO2/NO mole ratio varies relatively widely corresponding to a load fluctuation. At a high load time, almost all NOx is NO and the amount of NO2 is small. On the other hand, at a low load time the amount of NO2 increases while the amount of NO is small.
Catalyst A of the conventional composition exhibits a high NOx removal rate when NO concentration is high and NO2 concentration is low. However, when NO2 concentration is high and NO concentration is low, the NOx removal rate decreases, so that the NOx removal rate in the case of high NO2/NO ratio is assumed in designing a denitrifier, so that the amount of catalyst increases, which is economically disadvantageous.
Catalyst B exhibits a high NOx removal rate regardless of variation in the NO2/NO ratio. However, catalyst B is more expensive than the conventional catalyst A. Therefore, to replace the conventional catalyst A with catalyst B by disposing of catalyst A is economically disadvantageous, so that some measures are demanded.
Also, when a denitrifier is already operating smoothly using catalyst A of the conventional composition in a state in which a decrease in catalyst performance is scarcely recognized and the catalyst can still be used continuously, measures for restraining the generation of an alarm about increased concentration of exhausted NOx caused by an increase in NO2 concentration at low load time are demanded while the existing denitrification catalyst is made use of to the utmost.